This program will deal broadly with cellular and molecular aspects of antigen recognition by thymus-derived lymphocytes (T cells) and with T-cell regulation of two immune responses: induction of cytotoxic T lymphocytes and diversification of variable domains of antibodies during primary antibody responses. Antigen recognition will be studied by preparing antisera by xenogeneic, allogeneic, and isogeneic immunization using cloned T cells as immunogens. Antisera will be evaluated for their ability to block in a clone-specific manner the function of the T cells used as immunogens. Active antisera also will be used to immunoprecipitate components in extracts of 125I-labeled cloned T cells in an effort to detect and characterize any molecules on the T-cell surface that may be involved in antigen recognition. In other studies of cytotoxic T lymphocytes, cell surface "effector" molecules that may be involved in the lysis of target cells will be identified and tested to see if they have any of several obvious enzymatic activities (amino acid esterase, etc.).